1. Technical Field
This disclosure relates to an image forming apparatus, and more particularly to an image forming apparatus for reading surface properties of a sheet of recording media to adjust and control image formation.
2. Description of the Background
One example of related-art image forming apparatuses having two or more of printing, copying, plotting, and facsimile functions is an inkjet recording device employing a liquid discharge recording method. The inkjet recording device includes a recording head to discharge droplets of a recording liquid such as ink to form an image on a recording medium such as a sheet while the sheet is conveyed.
Examples of the inkjet recording device include a serial-type image forming apparatus, in which the recording head discharges liquid droplets while moving in a main scanning direction to form an image on the sheet, and a line-type image forming apparatus including a line-type recording head to discharge liquid droplets that does so without moving to form an image on the sheet.
In recent years, higher image quality and higher image accuracy are demanded for the image forming apparatuses. For example, a maximum resolution of from 4,800 to 9,600 (horizontal) x from 1,200 to 2,400 (vertical) dpi is set in widely-used inkjet recording devices, and an image is formed with dot intervals of from about 10 to 20 μm in a direction of sheet feed.
In order to achieve such higher image quality and higher image accuracy, higher accuracy in conveyance of a recording medium (or a sheet) when an image is formed thereon is required. Because an eccentricity of several μm is inevitably generated in a conveyance roller to convey the sheet due to cost and manufacturing reasons, an error of several μm in conveyance of the sheet occurs. In a serial-type inkjet recording device, vibration of the device due to reciprocal movement of the carriage that supports the recording head causes vibration of an encoder that detects an amount of rotation of the conveyance roller, an amount of movement of the carriage, and so forth, in a main scanning direction. Consequently, in a case in which timing of discharge of liquid droplets is controlled based on a result detected by a main scanning encoder, image deterioration occurs in the main scanning direction.
To solve the above-described problems, Published Unexamined Japanese Patent Application No. 2007-217176 (hereinafter referred to as JP-2007-217176-A) discloses a controller and a liquid ejection device in which surface characteristics of a sheet are photographed as a consecutive image while the sheet is conveyed by conveyance means. Multiple still images having different timings are extracted from the consecutive image thus photographed and compared with one another to calculate an amount of conveyance of the sheet, so that operation of the conveyance means is controlled based on the amount of conveyance of the sheet thus calculated.
In another approach, JP-2007-216648-A discloses a correction method and a compensation apparatus in which surface characteristics of a sheet are consecutively photographed while the sheet is conveyed by conveyance means. Multiple still images having different timings are extracted from the image thus photographed and compared with one another to calculate an actual amount of conveyance of the sheet. An operational amount of the conveyance means is measured to calculate an estimated amount of conveyance of the sheet based on the operational amount of the conveyance means thus measured. Thereafter, a corrective value of the operational amount of the conveyance means is calculated based on a difference between the estimated amount of conveyance of the sheet and the actual amount of conveyance of the sheet, and the operational amount of the conveyance means is corrected based on the corrective value thus calculated.
In yet another approach, JP-2007-254094-A discloses a paper carrying device in which an optical sensor including an LED or the like provided at a predetermined position along a conveyance path of a sheet periodically reads light and dark patterns in a certain area on a surface of the sheet while the sheet is conveyed. The same portion in the multiple light and dark patterns thus periodically read is compared to calculate an amount of positional change of the sheet.
However, in the above-described methods and devices, the amount of conveyance of the sheet is corrected based on readings from an optical sensor fixed to the devices. Specifically, relative positions of a carriage (or a recording head) and the sheet are indirectly measured via the optical sensor fixed to the devices. Consequently, when a relative amount of movement between the optical sensor fixed to the devices and either the carriage or the sheet varies due to vibration of the devices caused by reciprocal movement of the carriage, an error in the amount of conveyance of the sheet arises that cannot be corrected.
The serial-type inkjet recording device generally includes a main scanning encoder including an encoder scale provided along a main scanning direction and an encoder sensor provided to a carriage. A timing of liquid droplet discharge is determined based on a signal from the main scanning encoder, and the recording head is driven to discharge the liquid droplets at that timing.
In the serial-type inkjet recording device, vibration of the carriage due to reciprocal movement of the carriage itself and vibration of the serial-type inkjet recording device due to reciprocal movement of the carriage causes vibration of the main scanning encoder that detects the amount of movement of the carriage. Consequently, the timing of discharge of the liquid droplets as determined by readings from the main scanning encoder varies, causing image deterioration in the main scanning direction.